1. Field of the Invention
The present invention relates to a nonaqueous electrolytic secondary battery and a positive electrode for a nonaqueous electrolytic secondary battery; in particular, to a nonaqueous electrolytic secondary battery having improved characteristics under continuous charging and a positive electrode for such a nonaqueous electrolytic secondary battery.
2. Description of Related Art
In recent years, a decrease in the size and weight of mobile terminal devices such as cellular phones, notebook computers, and personal digital assistants (PDAs) has been rapidly achieved. With such a trend, there has been a demand to further increase in the capacity of batteries serving as driving power supplies of such mobile terminal devices. Lithium-ion batteries, which are configured to be charged and discharged through migration of lithium ions between the positive electrode and the negative electrode, have high energy density and high capacity. Accordingly, lithium-ion batteries are widely used as driving power supplies of such mobile terminal devices.
As mobile terminal devices have been made to have greater functionality, such as the function of replaying videos or the function of letting users play games, there has been a trend toward a further increase in the power consumption of the mobile terminal devices. Thus, there has been a strong demand for a further increase in the capacity of batteries. To increase the capacity of a nonaqueous electrolytic secondary battery, in addition to a method for increasing the capacity of the active material and a method for increasing the amount of the active material packed per unit volume, there is a method for increasing the charging voltage of the battery. However, when the charging voltage of a battery is increased, the electrolytic solution of the battery tends to decompose. In particular, when such a battery is stored at high temperature or is continuously charged, the electrolytic solution decomposes to generate gas, which swells the battery or increases the internal pressure of the battery. In consideration of such problems, the following batteries have been proposed.
Japanese Published Unexamined Patent Application No. 2007-242303 (Patent Document 1) proposes a battery in which a lithium composite oxide represented by LixM1−yLyO2 where L represents an element selected from the group consisting of the rare-earth elements, group IIIb elements, and group IVb elements has been treated with a coupling agent and remaining bonding groups of the coupling agent without being bonded to the lithium composite oxide have been inactivated. Patent Document 1 states that such a battery has an enhanced cycling characteristic when the battery is intermittently cycled under an environment at a high temperature.
In addition, to achieve the same object as in Patent Document 1, Japanese Published Unexamined Patent Application No. 2007-18874 (Patent Document 2) proposes a battery in which a lithium composite oxide represented by LixM1−yLyO2 where L represents an element selected from the group consisting of the rare-earth elements, group IIIb elements, and group IVb elements is used; the surface layer of the active material contains at least one element Le selected from the group consisting of Al, Mn, Ti, Mg, Zr, Nb, Mo, W, and Y; and the active material has been surface-treated with a coupling agent.
Furthermore, Japanese Published Unexamined Patent Application No. 9-199112 (Patent Document 3) proposes a battery in which the positive electrode mixture and the like contain an aluminum coupling agent. Patent Document 3 states that, as a result, the adhesion of the positive electrode is enhanced and hence a cycling characteristic of the battery can be enhanced.